This invention relates to detecting the fluid particle velocity of flow variations that are produced in a column of drilling fluid, i.e., drilling mud, by downhole signaling apparatus that is located in the region of a drill bit that is engaged in drilling a borehole in the earth.
During drilling of a well, it is desirable to provide information to the driller as to conditions that exist at the bottom of the well. For example, the inclination of the lower portion of the drill string with respect to a vertical reference axis and the azimuthal direction of that inclination are quite important, particularly in directional drilling, in order to assure that the borehole is drilled along an intended path. Similarly, it may be desirable to convey to the surface of the earth information relative to temperature and pressure conditions at the bottom of the hole, the weight that is applied to the bit at a particular instant, and other parameters that are important to the satisfactory completion of the drilling operation.
In the past, much of this downhole information has been obtained by parameter measuring instruments that were lowered into the drill string on a wire line. In many applications it is desirable to avoid the use of a wire line, if at all possible. Accordingly, systems have been devised for conveying information from a downhole location to the surface of the earth by employing pressure pulses, or transient acoustic waves, in the drilling fluid. These pressure pulses propagate upwardly through the column of drilling fluid to the surface where they are detected by a pressure responsive transducer. In some instances, a negative pressure pulse is produced by momentarily bypassing some of the circulating fluid from the interior of the drill string to its exterior. In other types of signaling equipment a positive pressure pulse is produced by closing or restricting a downhole valve through which drilling fluid flows toward the bit. In either case, the surface equipment responds to the momentary variation in drilling fluid pressure that results from actuation of a valve at the downhole location.
One reason that the detection of signaling fluid pressure pulses on the surface has been less successful than hoped is because the drilling fluid line at a well site includes an accumulator or desurger device whose function is to smooth out the pressure surges in the line caused by the piston strokes of the pump. Not only does the accumulator attenuate the pump pressure pulses, but it also attenuates the signaling pressure pulses that are produced downhole. I have found that the sensing method and apparatus of my invention results in optimum detection of signaling pulses in the drilling fluid and completely obviates the problem that limits the usefulness of prior art mud pulse pressure sensing.